Aluminum alloy powder product



2,963,780 Patented Dec. 13, 1960 2,963,780 ALUMINUM ALLOY POWDER PRODUCT John P. Lyle, Jr., and Raymond J. Towner, New Kensington, Pa., assignors to Aluminum Company of America,

This invention relates to aluminum alloy powder products, and it is more particularly directed to hot worked dispersion hardened compositions adapted for service at elevated temperatures.

Powdered metal products having a ferrous or cuprous base have achieved considerable commercial importance in recent years. The development of aluminum powder products has lagged behind that of other metals, partly because of the difliculty in making acceptable articles at a reasonable cost. Until recent years the major emphasis was placed on mixtures of aluminum and other metals which could not be produced conveniently by the usual melting and casting practices. For example, US. Patent 1,944,183 describes a compressed powder product cnsisting of to 80% aluminum and to 90% silicon. In this instance, the large proportion of silicon imparts properties which are characteristic of that element, since the composition is essentially a physical mixture of the two components.

Another type of aluminum powder product has been introduced in recent years which is made from oxidecoated aluminum flakes, an example of such a product being described in Swiss Patent 259,878. These products have a strength and hardness which exceeds that of commercially pure aluminum, especially at elevated temperatures. These properties appear to be developed by the uniform distribution throughout the metal matrix of finely divided oxide particles derived from the oxide coating on the flakes. Hardening of the aluminum matrix in this manner is referred to as dispersion hardening and is distinguished from the hardening produced by cold working or by precipitation of a constituent from a supersaturated solid solution.

While the flake powder products have attractive prop erties, the manufacture of the oxide-coated flakes presents some difficulties, particularly those associated with milling of the flakes in a ball mill or similar device. It is necessary either to mill the flakes in a dry or nearly dry condition or to follow the conventional practice of employing mineral spirits with or without a lubricant, such as stearic acid, and subsequently drying the mass of flakes. In addition to such ditficulties, the handling of the dry powder and the charging of it to a container for compaction presents some dangers and inconvenience.

An alternative and cheaper method of making comminuted aluminum is that known as atomization, wherein the aluminum is blown from a specially designed nozzle into a gas-filled chamber and the molten particles are frozen almost immediately. The solid particles thus produced tend to be spheroidal in shape, are free from any lubricant, and a given volume of the particles has a higher apparent density than the same volume of flake powders. It has been found, however, that such atomized aluminum powder yields compacted powder products which have inferior high temperature strength and hardness compared to those made from fiaketype of powder.

One of the objects of our invention is to provide a but worked powder product made from atomized aluminum alloy powder, the product being characterized by a high strength at elevated temperatures. Another object is to provide a hot worked aluminum alloy atomized powder product which does not require a preliminary thermal treatment to develop a high strength at elevated temperatures. Still another object is to provide a but worked atomized aluminum alloy product which retains its strength at elevated temperatures even upon long exposure. A further object is to provide a hot worked atomized aluminum alloy powder product that retains to a substantial degree at elevated temperature any effect of cold working produced during fabrication of the product. Another object is to provide a hot worked atomized aluminum alloy powder product which is of the dispersion hardening type, but which does not depend upon the presence of oxide particles to impart strength and hardness at elevated temperatures. Another object is to provide a hot worked atomized aluminum alloy powder product which has a higher modulus of elasticity than pure aluminum.

These and other objects are achieved in a compacted and hot worked aluminum alloy product, which is free from aluminum oxide except as an incidental impurity, and made from atomized particles of an alloy consisting of at least aluminum and from 2.5 to 20% by weight of iron as the essential components, and preferably from 5 to 10% of iron. In these particles, as initially produced by the atomizing process, the iron is present in the form of a substantially insoluble constituent such as FeAl in the binary alloy. Under the microscope the constituent generally appears in the form of dendrites and/or as a component of a eutectic or peritectic product. In all cases the iron constituent has an extremely fine size, the dendrite stems and branches having a thickness of not over 1 micron, and usually less than 0.4 micron while in the eutectic or peritectic prodnet, the longest dimension generally does not exceed 1 micron and usually it is less than 0.4 micron. These dimensions are much smaller than those which characterize the constituent as it appears in conventional castings. Upon hot working the atomized powder compacts, the size of the iron constituent particles is reduced to less than 0.4 micron, generally between 0.03 and 0.3 micron. It has been found to be essential to observe these dimensions in order to secure a high strength and hardness at elevated temperatures. Since iron is virtually insoluble in solid aluminum, only 0.05% being soluble at the eutectic temperature, the composition is considered to be of the dispersion hardening type. Moreover, there is no intentional addition of oxide or oxide coated particles to increase the strength and hardness, the properties of our hot worked product being dependent upon the size and distribution of the iron constituent.

. 1 3 it has been found that our product possesses a minimum tensile strength of 15,000 p.s.i. and a minimum yield strength of 12,000 p.s.i. at 600 F. after an ex' posure of 100 hours to that temperature and that these strength values remain substantially unchanged upon longer exposure at that temperature. Also, at 800 F. the minimum tensile strength is 8,000 p.s.i. and the yield strength 5,000 p.s.i. after an 100-hour exposure. The hot worked composition possesses still another advantageous property, namely, it exhibits substantially no 'recrystallization upon being heated to elevated temperatures. This resistance to recrystallization perhaps accounts in part for the retention of strength at elevated temperatures. Articles made from this product are useful for cylinders and pistons in piston type engines, and for spacers, after burnercontrols and other parts of jet engines.

The fine aluminum alloy particles are produced by the atomization process which yields very small substantially equi-axed bodies having a cast structure. The process must be controlled in such a manner as to produce particles of a size such that the majority of them will pass through a 200 mesh scheen (74 micron opening) and that none or only va small proportion are larger than 100 mesh (145 micron opening). Also, the atomization process must be set up so that the molten metal spray is so drastically chilled that the desired size of iron-containing constituent is produced. As mentioned above, the thickness of the constituent in the chilled particles must be under one micron and, preferably, under 0.4 micron. This size is much smaller than that of the particles of iron constituent found in cast alloys or even in sintered powder mixtures of aluminum and iron having the same chemical composition.

The hot worked powder product may be made by first forming a compact and working it or in some cases, such as extrusion, the entire operation may be carried out in one press in a more or less continuous manner. In any case, the atomized alloy powder, either in a cold or a preheated condition, is charged to a compression chamber, the powder heated to a hot working temperature preferably between 700 and 900 R, if the powder is not already at such a temperature, and compressed. The pressure exerted will depend upon several factors such as the composition of the alloy, the mass being compressed and the temperature of which compression is effected. Generally, a pressure of 30,000 to 120,000 p.s.i. is adequate to form a suitable compact. The compact may be immediately hot worked, as by extrusion, or it may be transferred to metal working apparatus, such as a forging press or a rolling mill. Also, if desired, the compact may be cooled to room temperature :and later reheated for hot working. When the compact is to be rolled or forged, it is preferred that the compact be first extruded in a suitable shape and then rolled or forged. The hot working of the compacted alloy powder is preferably done within the temperature range of 700 to 900 F.

As a consequence of such working, the size of the ironcontaining constituent is further reduced thereby producing an extremely fine, uniform dispersion of the constituent. The hot worked product may be handled or finished according to conventional practices, including cold'working, if desired. To facilitate further oold working, it may be desirable to heat the product to a sufiiciently high temperature to produce recovery rather than recrystallization.

The strength of the hot worked product :atelevated temperatures is affected by the-sizev of theiron constituent, as mentioned above. In general,-the strength increases 4 as the size of the constituent decreases. Furthermore, a finer dispersion of the constituent usually exists in the smaller sized atomized particles than in the larger sized particles. To achieve the highest strength, it is there fore desirable to employ the finest size of particles. Although particles as large as mesh microns) can be used, we prefer to employ particles of 200 mesh (74 microns) and finer. We haveiound that particles passing through a 325 mesh screen (43 microns opening) yield products having the highest strength, usually not less than 18,000 p.s.i. in tensile and 14,000 p.s.i. in yield strength at 600 F. 1

To supplement the etfectof the iron, and in some cases to increase the strength of the hot worked product, it may be desirable to add at least one hardening element of the group consisting of 0.1 to 10% manganese, 0.1 to 10% nickel, 0.1 to 10% cobalt, 0.1 to 10% chromium, 0.1 to 10% titanium, 0.1 to 10% zirconium, and 0.1 to 10% vanadium. Where these are employed, the total amount should not exceed 10%, :and preferably the iron content should exceed that of the other elements. The foregoing elements appear to interact with the iron in such a manner that their solubility is actually less than it is in the binary alloys of these elements with aluminum. Such a reduction in solubility is advantageous in improving the strength of the hot worked product. The alloys containing these elements are prepared by forming a melt of the alloy and atomizing it asdescribed above. By controlling the atomization of the molten alloy the desired size of insoluble iron-containing constituent is obtained. These constituents should not exceed one micron in size and, preferably, not over 0.4 micron in thickness in the atomized particles.

Another desirable property of products fabricated from our atomized alloy powders is the high modulus of elasticity that is attainable through selection of the amount of iron and the supplemental element added to the aluminum-iron base. Wrought commercially pure aluminum has a modulus of 9,900,000 p.s.i., but a hot worked powder product consisting of aluminum and 7.6% iron was found to have a modulus of 11,400,000 p.s.i. while a similar product containing 12.8% iron had a modulus of 12,700,000 p.s.i. The addition of certain elements that apparently form intermetallic compounds is also beneficial in increasing the modulus. For example, a hot worked powder product composed of aluminum, 5.5% iron and 5% chromium had a modulus of 12,500,000 p.s.i. In the light of these examples, it will be appreciated that a high modulus can be achieved along with a high strength.

The tensile properties that can be developed in binary aluminum-iron powder compositions are illustrated in the following examples. The iron was added to molten aluminum and the resulting alloy atomized to produce finely divided cast particles, the most of which passed through a 200 mesh screen (74 micron opening). The atomized powder of each composition was placed in the cylinder of an extrusion press, heated to about 800 F. and compressed under a pressure of about 100,000 p.s.i. against a blind die to form a compact about 7 inches in length and 4% inches in diameter. The compact was either removed from the cylinder, heated to 800 to 850 IF., and extruded to a rod inch in diameter, or the blind die was replaced by an extrusion die rand the compact extruded immediately into inch diameter rod. Tensile test specimens were cut from the rod, one portion was heated to 600 F. for 100 hours and tested; a second portion was heated at the same temperature for 1,000 hours, and tested and a thirdportion was heatedto 800? F. for a 100-hour period and tested. The composition l r r 1 The beneficial effect of decreasing the particle size of the atomized alloy powder on the strength of the ex- TABLE I Tensile properties of extruded Al-Fe powder products 100 Hrs. at 600 F. 1,000 Hrs. at 600 F. 100 Hrs. at 800 F. Alloy Percent Fe Tensile Yield Percent Tensile Yield Percent Tensile Yield Percent Strength, Strength, Elong. Strength, Strength, Elong. Strength, Strength, Elong.

p.s.i. p.s.i. p.s.i. p.s.i. p.s.i. p.s.i.

A 4. 3 17,800 15, 100 23 17, 500 14, 600 21 8, 200 6, 300 31 B 5. 3 20, 800 17, 500 12 20, 400 16, 300 14 8, 400 0, 400 24 C 1 7. 6 21, 900 17,800 19 21, 900 17,700 19 10, 100 7,800 28 D 12. 8 25, 300 19, 400 7 27, 900 22, 500 6 12, 200 8,300 13 It isto he noted that at 600 F. the tensile strength of all the compositions exceeded 15,000 p.s.i. while the yield strength was above 13,000 p.s.i. Furthermore, the

longer exposure at 1000 hours caused but slight change a in the properties, thus indicating that at a constant ternperature little change in properties occurs over a relatively long period of time. The foregoing property values may be compared with those of a commercial aluminum forging alloy designed for service at elevated temperatures having a nominal composition of 4% copper, 0.6% magnesium, 2% nickel and the balance aluminum. In the solution heat treated and precipitation hardened condition, forgings of this alloy have a typical tensile strength at 600 F. of 5,500 p.s.i., a yield strength of 3,000 p.s.i. and an elongation of 60%. It is also significant that even at 800 F. the tensile and yield strengths of the extruded aluminum-iron specimens. exceeded those of the forged alloy at 600 F. It is therefore evident that the aluminum-iron atomized powder products possess 3r 0 some exceptional properties at high temperatures.

Examination of the atomized particles and extruded product under the light microscope, electron microscope and by X-ray difi raction revealed that in. the atomized particles the iron constituent had a thickness in the approximate range of 0.03 to 0.4 micron and that in the extruded product the constituent size was smaller, on the order of 0.03 to 0.3 micron. Further, it was determined by microscopic and X-ray d-iifraction examinations that the extruded material had not recrystallized during exposure to the elevated temperatures, even at 800 F.

The effect of added elements upon properties of the base aluminum-iron composition is to be seen in the following examples. The various. alloys were prepared,

atomized, compacted and extruded in the same manner F The tensile test specimens were a as described above. heated to 600 F. for 100 hours and tested. The composition and tensile properties of the materials are given in Table II below.

truded product is illustrated by the examples in Table III below, where the tensile and yield strengths at 600 F after a 100-hour exposure increase as the particle size obtained by means of the control screens becomes smaller.

TABLE III Efiect of powder particle size on tensile properties at 600 F. of extruded alloy powder products Powder Particle Size Percent Tensile Yield Percent US Sieve N 0. Fe Strength, Strength, Elong.

p.s.i. p.s.i. in 4D 0 least one hardening element selected from the group composed of 0.1 to manganese, 0.1 to 10% nickel, 0.1 to 10% cobalt, 0.1 to 10% chromium, 0.1 to 10% titanium, 0. to 10% zirconium and 0.1 to 10% vanadium, the total amount of said elements not exceeding 10%, all percentages being by weight.

3. A hot worked aluminum base alloy powder article free from aluminum oxide except as an incidental impurity, and formed from atomized powder of an aluminum base alloy containing at least 70% by weight aluminum and from 2.5 to 20% by weight iron as the essential components, the iron-containing constituent in the hot worked article being present in the form of finelly divided uniformly distributed insoluble particles having a maximum TABLE II Tensile properties of modified aluminum-iron COMPOSZIZOHLS at 600 F.

Percent Percent Percent Percent Percent Percent Percent Percent Tensile Yield Percent Alloy Fe Mn Ni Cr 00 V Ti Zr Strength, Strength, Elong.

p.s.i. p.s.i.

E 3. 9 18. 400 12, 800 22 F 4. 5 20, 900 14, 500 15 G 4. 5 19, 400 14, 500 23 H 3. 2 15,800 12,100 25 I. 5. 5 5.0 27,100 20, 300 4 I- 3.6 1.0 20. 500 17, 400 10 K 5. 6 0.5 22, 500 17, 500 10 L.-. 2. 5 0. 4 20, 500 16, 700 18 M 7. 8 0.2 24, 300 20, 800 16 It is evident that the addition of other elements has served to increase the strength of the basic composition with the exception of manganese. However, this element appears to be beneficial when combined with other elements. It is also noteworthy that even small amounts of added elements, as in alloy M, have a marked effect and increase the strength substantially.

a 4. A hot worked aluminum base alloy powder article free from aluminum oxide except as in incidental impurity and formed from atomized alloy powder particles less than 100 mesh in size, said alloy containing at least 70% by weight aluminum and 2.5 to 20% by weight iron as the essential components, the iron-containing constituent in said powder particles being finely divided and having a maximum thickness of 1 micron prior to hot working,

the iron-containing constituent also being finely divided in the hot worked-article and having a maximum constituent size of 0.4 micron, said alloy being substantially free from elements which form solid solutions with aluminum,

except as they occur as impurities, said hot worked article '8 and formed from atomized alloy powder particles less than 325 mesh in size, said alloy containing atfleast 70% by weight aluminum and 2.5 to 20% by weight iron as the essential components, the ironcontaining constituent in the hot Worked article being present in the form of finely divided uniformly distributed insoluble particles having a maximum thickness of 0.4 micron, said alloy being sub- 1 stantially free from elements which form solid solutions with aluminum, except as they occur as impurities, said hot worked article being characterized in the as-worked condition by a tensile strength at 600 F. after a 100- hour exposure of not less than 18,000 psi. and a yield strength of not less than 14,000 psi.

References Cited in the file of this patent UNITED STATES PATENTS Jones June 23, 1942 Ennor Oct. 15, 1957 Patent N0. 2,963,780 December 13, 1960 John P. Lyle, Jra, et al.,

It is hereby certified that er ent requiring correction and that t corrected below,

ror appears in the above numbered pathe said Letters Patent should read as Column 6, line 43, for "0." read 0.1

for finelly" read finely column 7, "in' read an ""o line 52, line 2, for

Signed and sealed this 13th day of June 1961,

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No.,, 2,963,780 December 13, 1960 John P. Lylle, Jr, et alo It is hereby certified that error eppears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 43, for "0," read 0.1 line v52', for "finelly read finely column 7, line 2 for "in read an Signed and sealed this 13th day of June 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

3. A HOT WORKED ALLUMINUM BASE ALLOY POWDER ARTICLE FREE FROM ALUMINUM OXIDE EXCEPT AS AN INCIDENTAL IMPURITY, AND FORMED FROM ATOMIZED POWDER OF AN ALUMINUM BASE ALLOY CONTAINING AT LEAST 70% BY WEIGHT ALUMINUM AND FROM 2.5 TO 20% BY WEIGHT IRON AS THE ESSENTIAL COMPONENTS, THE IRON-CONTAINING CONSTITUENT IN THE HOT WORKED ARTICLE BEING PRESENT IN THE FORM OF FINELY DIVIDED UNIFORMLY DISTRIBUTED INSOLUBLE PARTICLES HAVING A MAXIMUM THICKNESS OF 0.4 MICRON, SAID ALLOY BEING SUBSTANTIALLY FREE FROM ELEMENTS WHICH FORM SOLID SOLUTIONS WITH ALUMINUM, EXCEPT AS THEY OCCUR AS IMPURITIES, SAID HOT WORKED ARTICLE BEING CHARACTERIZED IN THE AS-WORKED CONDITION BY A TENSILE STRENGTH AT 600*F. AFTER A 100-HOUR EXPOSURE OF NOT LESS THAN 15,00 P.S.I. ANDA YIELD STRENGTH OF NOT LESS THAN 12,000 P.S.I. 